From Dragon Mania Legends (DML) Wiki
Emerald Energy is the energy used each time a battle is fought in Emerald Wars, a Clan Event. Every player has 6 at maximum as a base amount, and attacking a mine costs 1 Emerald Energy. If a Trainer's Dragon participates in a battle for control of an emerald mine, it needs to rest for a certain period before it can fight again (it can be used outside the event while resting). The resting time can be skipped using Revitalization Tokens earned completing regular Clan Quests.
Emerald Energy automatically regenerates at 1 every 60 minutes, up to the maximum base amount. It can also be recovered through Revitalization Tokens (at the cost of 1 Revitalization Token per energy), through Gems (at the cost of 5 Gems per energy), or through leveling up. Both methods fully replenish the energy, and there is no option to select the amount of energy to replenish. The only way to replenish smaller amounts of energy is to use small amounts at a time, and then replenish the amount remaining to maximum.
Emerald Energy is refilled by either pressing the + button on the Emerald Wars main screen, or by trying to engage a battle without any energy left (which opens the refill screen, where energy can be refilled using Revitalization Tokens or Gems).
See Battle Energy for more information.